Immunogens will be prepared by coupling acidic derivatives of naltrexone and 6 Beta-hydroxynaltrexone with bovine serum albumin. The immunogen will be administered subcutaneously to rabbits and the sera collected after 4-6 weeks. The initial coupling reaction will be made at different sites on the molecules such that cross-reactivity of the two antibodies with the reagents should be minimal. Analyses of serum concentrations and urine concentrations of naltrexone and its reduction product will be done in animals and the time course plotted. Urines will be assayed by pharmacological methods to confirm the accuracies of the radioimmunoassays.